LTE Physical Structure
A 3GPP supports a type 1 Radio Frame Structure that is applicable to an FDD (Frequency Division Duplex) and a type 2 Radio Frame Structure that is applicable to a TDD (Time Division Duplex).
FIG. 1 illustrates the structure of a type 1 radio frame. The type 1 radio frame consists of 10 subframes, and each subframe consists of 2 slots.
FIG. 2 illustrates the structure of a type 2 radio frame. The type 2 radio frame consists of 2 half frames, and each half frame consists of 5 subframes, a DwPTS (Downlink Pilot Time Slot), a Guard Period (GP), and an UpPTS (Uplink Pilot Time Slot). Herein, one subframe consists of 2 slots. The DwPTS is used for initial cell search, synchronization, or channel estimation performed by a user equipment. The UpPTS is used for channel estimation performed by a base station and for uplink transmission synchronization performed by the user equipment. The guard period corresponds to a period for eliminating interference occurring in an uplink due to a multiple path delay of a downlink signal between an uplink and a downlink. Meanwhile, regardless of the type of the radio frame, one frame is configured of 2 slots.
FIG. 3 illustrates a slot structure of an LTE downlink. As shown in FIG. 3, a signal being transmitted from each slot may be expressed by a Resource Grid, which consists of NRBDL NSCRB number of subcarriers and NsymbDL number of OFDM (Orthogonal Frequency Division Multiplexing) symbols. Herein, NRBDL represents a number of Resource Blocks (RBs) within a downlink, NSCRB represents a number of subcarriers configuring one RB, and NsymbDL represents a number of OFDM symbols included in a downlink slot.
FIG. 4 illustrates a slot structure of an LTE uplink. As shown in FIG. 4, a signal being transmitted from each slot may be expressed by a Resource Grid, which consists of NRBUL NSCRB number of subcarriers and NsymbUL number of SC-FDMA (Single Orthogonal Frequency Division Multiplexing Access) symbols. Herein, NRNUL represents a number of Resource Blocks (RBs) within an uplink, NSCRB represents a number of subcarriers configuring one RB, and NsymbUL represents a number of SC-FDMA symbols included in an uplink slot.
A Resource Element is a resource unit that is defined by indexes (a, b) within the downlink slot and the uplink slot. Herein, a indicates an index within a frequency axis, and b represents an index within a time axis.
FIG. 5 illustrates the structure of a downlink subframe. Referring to FIG. 5, in a subframe, a maximum of 3 OFDM symbols located at the beginning of a first slot correspond to a control region allocated to a control channel. The remaining OFDM symbols correspond to a data region allocated to a Physical Downlink Shared Channel (PDSCH). Examples of a downlink control channel used by a 3GPP LTE may include a PCFICH (Physical Control Format Indicator Channel), a PDCCH (Physical Downlink Control Channel), a PHICH (Physical Hybrid ARQ Indicator Channel), and so on.
The PCFICH is transmitted from a first OFDM symbol of one subframe, and the PCFICH transmits information related to a number of OFDM symbols that are used for transmitting a control channel within the corresponding subframe. As a response to an uplink transmission, the PHICH transmits an HARQ ACK (Acknowledgement)/NACK (Negative Acknowledgement) signal. The control information being transmitted through the PDCCH is referred to as DCI (Downlink Control Information), which may include uplink or downlink scheduling information or information on uplink transmission power control commands on random UE groups. The PDCCH may carry and deliver transmission format information, resource allocation information of a Downlink Shared Channel (DL-SCH), paging information within a PCH (Paging Channel), system information within the DL-SCH, resource allocation information on higher layer control messages, such as random access responses being transmitted over the PDSCH, a group of transmission power control commands on individual UEs within random UE groups, information on transmission power control commands, information on the activation of a VoIP (Voice of Internet Protocol), and so on. Multiple PDCCHs may be transmitted with the control region. The UE may monitor multiple PDCCHs. Herein, a PDCCH is transmitted by one or multiple groups of consecutive Control Channel Elements (CCEs). The CCE refers to a logical allocation unit, which is used for providing a coding rate to the PDCCH based upon the state of the corresponding wireless channel. Herein, a CCE corresponds to multiple resource element groups. The format of a PDCCH and a number of available bits in the PDCCH may be decided in accordance with a correlation between the number of CCEs and the coding rate that is provided by the CCE. Then, the base station decides the PDCCH format based upon the DCI that is transmitted to the UE, and, then, the base station attaches (or adds) a CRC (Cyclic Redundancy Check) to the control information.
The CRC is masked with a unique identifier (i.e., Radio Network Temporary Identifier (RNTI)) in accordance with the usage or owner of the PDCCH. If the PDCCH is specified for a specific UE, a unique identifier (e.g., C-RNTI (Cell-RNTI)) is masked by using the CRC. And, if the PDCCH is specified for a paging message, a paging indicator identifier (e.g., a Paging-RNTI (P-RNTI)) is masked to the CRS. Also, if the PDCC is specified for the system information (more specifically, hereinafter referred to as a System Information Block (SIB)), a system information identifier and a system information RNTI (S-RNTI) may be masked to the CRC. Also, in order to display a random access response, which corresponds to a response of the UE to the transmission of a random access preamble, a random access RNTI (RA-RNTI) may be masked to the CRS.
FIG. 6 illustrates a structure of an uplink subframe. As shown in FIG. 6, within a frequency domain, an uplink subframe may be divided into a control region and a data region. The control region is allocated to a PUCCH (Physical Uplink Control Channel) for transmitting uplink control information. The data region is allocated to a PUSCH (Physical Uplink Shared Channel) for transmitting data. In order to maintain the attributes of a single carrier, the UE does not transmit the PUCCH and the PUSCH at the same time. The PUCCH for one UE is allocated as an RB pair within one subframe. The RBs included in the RB pair each occupies a different subcarrier within two slots. It may be said that the RB pair allocated to the PUCCH performed frequency hopping at a slot boundary.
A multiple-carrier system or a carrier aggregation system refers to a system aggregating one or more carriers having a target bandwidth smaller than a target bandwidth, when configuring a broadband that is targeted in order to support a broadband. When aggregating one or more carriers each having a bandwidth smaller than the target bandwidth, the bandwidth of the aggregated cattier may be limited to the bandwidth uses in the conventional system in order to provide backward compatibility with the conventional IMT system. For example, the conventional 3GPP LTE system supports bandwidths of 1.4, 3, 5, 10, 15, and 20 MHz, and, in an LTE-A (LTE-Advanced) system, which is an enhanced (or evolved) version of the LTE system, by using only the above-described bandwidths supported by the LTE, an bandwidth larger than 20 MHz may be supported. Also, regardless of the bandwidth uses in the conventional system, a new bandwidth may be defined so as to support carrier aggregation.
In the conventional system, a one-to-one correspondence between uplink carriers and downlink carriers is defined in advance for each frequency band. However, in the one-to-one correspondence is not established between the uplink carriers and the downlink carriers, a problem may occur in the operations associated with the carrier identifier. Therefore, a method for resolving such problems is required.